1. Field of the Invention
The present invention relates to a shaft for a fluid dynamic bearing device, which has an external circumference shaped to generate a fluid dynamic in a clearance to a bearing when rotated relatively to the bearing, a fluid dynamic bearing device having the shaft, and a method of manufacturing the shaft.
2. Description of the Related Art
In a certain type of fluid dynamic bearing, grooves are formed in the direction across the sliding direction in at least one of a bearing surface of a shaft and a bearing, to improve the stability of the axis of rotation. The grooves are available in two types, the herringbone type (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2000-227119), and the type arranged parallel to the axis of rotation (e.g., International Pat. Appln. KOKAI Publication No. 98/38433).
Another type of dynamic air pressure bearing device has a fixed shaft, and a hollow rotation shaft fit with clearance to the external circumference of the fixed shaft. The external circumference of the fixed shaft is shaped to be an equal-radius multi-arc, or equal-radius ellipse. A gap between the external circumference of the fixed shaft and the internal circumference of the hollow rotation shaft is shaped like odd numbers of (three) sine waves. This type is used for an optical deflector. (For example, Jpn. Pat. Appln. KOKAI Publication No. 7-230056.) The surface of the shaft is anodized.
A fluid dynamic bearing device and a shaft for a fluid dynamic bearing according to the invention is a rotating body, which rotates one way or two ways (forward and reverse) at a low speed or a high speed, or rotates continuously or intermittently, or a member which supports the rotating body. The rotating body or the member supporting the rotating body has a function of moving a lubricating medium in the circumferential or longitudinal direction by rotation, and stabilizes rotation by decreasing sliding resistance.
It is necessary to increase the number of grooves and form the grooves more precisely for increasing the rotation accuracy of a fluid dynamic bearing device to the extent that generation of a half whirl in a high rotation frequency range of the fluid dynamic bearing device is controlled, and the shaft and bearing are stably rotated even at a low rotation frequency without colliding with each other. The herringbone grooves are formed by etching, shotblasting or shot-peening a masked rod-shaped material to reduce the machining cost. In these machining processes, it is difficult to increase the machining accuracy above than a certain level. As the rotation characteristics differ depending on the direction of the herringbone pattern, it is required to take account of the rotating direction when assembling the bearing device.
When grooves are formed in the direction along the axis of rotation by etching, shotblasting, shot peening, grinding or laser machining, a discontinued part is formed in the boundary between the grooves and the external circumference. Thus, when the rotation frequency increases, a separation layer easily occurs in the downstream side of the rotation from the discontinued part. Formation of a separation layer may cause a failure to generate a desired fluid dynamic between the shaft and the bearing.
Further, in any machining method, a process to increase the circularity of shaft and bearing is necessary in addition to a process of forming grooves. A shaft exposed to the processes of forming grooves and increasing the circularity must exhibit the machining and positioning accuracy with high reproducibility in each process. Thus, it is difficult to decrease the cost of machining the shaft.